Keep getting +1 tooth when cutting worm gear wheels??

[niggle21]

I'm trying to make a 120 tooth worm gear wheel using a M14x2 tap in the lathe as a gear hob, the gear blank is mounted on a spindle on the cross-slide and driven by the tap / hob as it cuts.

Everything's working fine but the problem i'm having is that I keep getting 121 teeth on the gears! This is how I worked it out..

2mm tooth pitch, 120 x 2 = 240mm circumference, so I worked backwards to get a diameter for the gear which came to 76.39mm. I turned the gear black to EXACTLY 76.39mm.

I tried this with three seperate gear blanks thinking that the tap might have not cut right but no luck..

I then tried turning a gearblank to 75.76mm, which is the diameter that I calculated for 119 teeth, and it comes out to 120 teeth! Perfect!

But WHY??

Can someone please explain to me why this is?


Thanks in advance,
Nigel.

 

[eromlignod]

...No, a gear has the same amount of "gaps" as it has teeth.

Part of your problem appears to be that you are cutting your blank to the pitch diameter (you should start with the OD). Technically, this should result in fewer teeth rather than more teeth, so I'm guessing that you also have miscalculated your index speed for the gear blank and/or the hob.

The OD would be

OD = (2 x center distance) - worm PD + (2 x addendum)

Don
Kansas City